Traveling wave tube having rectangular waveguide slow wave structure with equiangularly spaced electron guns



United States Patent O 3 l5@ 283 TPAVTJLlN-G WAVE HAK/L Wl RLJTANGU- LA?. 'WAV'.GlUlDE f? WAVE STRUC'FJR 'TH EQUAIS SAQEB Glllsl oseph Rowe, fisher, Mich., assigner to the States of America regresented by the Secretary of the .il Force Filed May 19, i951, Ser. No, lllllll l Claim. (Cl. SiS-3.o)

This invention relates to traveling wave tubes of the type employing an open sided helical rectangular waveguide as a slow wave structure and has as its object an increase in the eihci ricy and po er capa -l .es of such tubes. ln prior art devices ot this t' the dimension of the waveguide in the radial direction is less than the outer radius, leaving space to pass the electron beam o the tube along the of the This construction inherently l its the coupling between tire beam and the electromagnetic wave in the waveguide with the result that the full power gain capabilities of this type ot g tube are not realized.

the anis oi the 1nelix and allowed to pass through aligned apertures in the walls of the 'veguide This p "ts a greater interaction bet vee the beam and the electromagnetic the slow wave structure and results in increased amplification and power output. Further, this construction permits a plurality of bearns to be used with a further improvement in the gain and power capabilities of the t be.

A more detailed description oi the invention will be given with reference to the specific embodiment t: ereor" shown in the accompanying drawing in which- FlG. 1 is a cross sectional View of the tube, and

2 shows alt rnative rneans for coupling to tie slow wave structure of the tube.

ReferrinCT to G. l, the tu e comprises an evacuated cylindrical metal envelope l within which is located a slow wave structure in the form or a helical rectangular waveguide with inner side open. Thi waveguide is formed by a metallhelical i bbon 2 having one edge in continuous electrical contact with the inner surface or" cylinder l. rl`he wi tb of the ribbon is made equal to the inner radius of the cylinder so that the waveguide wall in the radial direction extends to the axis of cylinder l. Au input coupling 3 is provided for introducing radio frequency energy into the slow wa 'e structure and an output coupling 4 for removing amplified radio frequency energy from the tube. These couplings are illustrated as loops but suitable coupling device may be used. An alternative form is shown in lJlG. 2 Where the coupling device is a rectangular waveguide 3 formed as an extension of the helical waveguide of the tube l'.

In the tube illustrated there are four electron beams provided by rour similar electron guns spaced 96 apart, three of which are shown in the ligure. These guns may be of any suitable type. ln the example shown the outer shell S, having a beam aperture 6, serves as the acceleratint7 electrode of the gun and is supported from conductive ring 7 by means of a conductive bracket 8. The remainder of the gun, not shown, is insulatedly supported inside shell and comprises the cathode and focusing electrodes. Conductor g connects the cathode through sealing and insulating bead lo to the negative terminal of direct current source ll. Conductors l2 passing through ice sealing and insulating beads lil carry heater current from source lll to the cathode heater. The body of the tube is connected through electron collectors l5, one for each beam, and conductor lo to the positive terminal of source ll. Since accelerating electrode S is at the saine potential as the body l, it is positive relative to the cathode of the gun by the amount of the voltage of source This voltage is made such as to give the required el ctron velocity. Each beam passes through a series of aligned apertures 17 in the ribbon 2 to its collector i5.

The operation of the device depends, as in other traveling wave tubes, on the conversion of the kinetic energy which the electrons have by virtue of their velocity into electromagnetic energy in the helical waveguide slow wave structure. The electron beams leave the electron guns to the lett of the input to the slow wave structure essentially unmodulated and enter the slow wave structure at the same end at which the radio frequency energy is introduced through the input coupling device 3. The axial co onent ot the electric rield exerts a force on the electrous in the beam, accelerating some and decelerating others, to produce a hunched beam. The hunched beam induces currents in the slow wave structure and, because the slow wave structure has impedance, a voltage is developed which adds to the ampaude of the voltage alon the helix, resulting in an amplification of the impressed radio requevcy wave. The amplified Wave is taken from output coupling device d.

in order for l e radio frequency wave and the electron beam to interact the velocity of the electrons must be slightly greater than the axial phase velocity of the wave on the slow wave structure. As the electrons travel down the length of the slow wave structure they gradually slow down due to the conversion o their kinetic energy into electromagnetic energi. rtherefore, it is desirable that the axial phase velocity or" the electromagnetic Wave in the helical waveguide be decreased at the saine rate that the velocity of tue electrons decrease in order to obtain the optimum relationship between wave velocity and electron velocity over the length of the slow Wave structure. Since the axial phase velocity is directly related to the pitch of the helix of the waveguide, this may be accomplished by gradually decreasing the pitch of the helical waveguide in approximately an exponential Inanner along the length ot the tube as shown in FlG. 1.

I claim:

A traveling wave tube compris'ng an evacuated rnetallic cylinder, a conductive helical ribbon having one edge in continuous electrical contact with the inner surtace of said cylinder and having a Width equal to the inner radius of said cylinder, said ribbon forming with the inner surface of said cylinder a helical waveguide slow wave structure, said ribbon having aligned apertures therein to form axially directed passageways, the axes of said passageways being parallel to and equally spaced from the axis of said cylinder and having equiangular spacing about the am's of said cylinder, means for passing a beam of electrons through each passageway, means for introducing high frequency enerffy into said slow Wave structure at the end at which the electrons enter said passageways, and means for removing amplified high frequency energy from said slow Wave structure at its other end.

Tiley Feb. 13, 1951 Warnecke Ian. 23, 1962 

